The invention relates to a vehicle steering system of the by-wire design type without any mechanical linkage between the driver's steering wheel and a steering gearbox, which initiates a steering movement of at least one steerable wheel of the vehicle, where the steering movement is specified by the driver by way of the steering wheel. An actuator, which is actuated by an electronic control unit with signals from the steering wheel input, can act on the steering gearbox. This steering system also has a hydraulic default level, which has a hydraulic feed unit, coupled to the steering wheel, and a hydraulic actuating unit, which is coupled to the steering gearbox or forms the same. The hydraulic feed unit and the hydraulic actuating unit can be connected together by way of two hydraulic lines in such a manner that when the actuator is non-functional, the steering wheel rotational angle input with the steering wheel is transformed into a corresponding wheel steering angle by use of the hydraulic feed unit and the hydraulic actuating unit. These two hydraulic lines between the hydraulic feed unit and the hydraulic actuating unit can be connected together by a switchable so-called steering valve. With respect to the relevant prior art reference is made, by way of example, to DE 198 01 393 A1.
The so-called steer-by-wire systems, in which the steering request input by the driver of the vehicle at the steering wheel or the like (instead of a steering wheel, a so-called side stick or the like can be provided as the so-called steering control element—for the sake of simplicity only one steering wheel is discussed below, and with respect to operating this steering wheel, the steering wheel rotational angle is discussed) is not transmitted to the steerable wheels of the vehicle by the direct mechanical route—that is, as in the case of the conventional modern vehicles by way of the so-called steering column or rather the steering spindle as well as a steering gearbox, coupled thereto (for example, a rack and pinion steering gearbox)—but rather by the electric or hydraulic route, are advantageous with respect to the configuration possibilities for the system components in the vehicle. With such steer-by-wire systems a steering angle can be adjusted, even independently of the driver's input, in an advantageous way at the steerable wheels, for example, to the effect that, as a consequence, an unstable driving condition is counteracted. Expressed in general terms, the steer-by-wire system does not exhibit a fixed correlation between a steering input, set at the steering wheel (=steering wheel rotational angle), and the steering movement (=wheel steering angle), executed at the steerable wheels of the vehicle on the basis of this steering input, so that it is possible to achieve in an advantageous manner a so-called variable steering ratio between the steering wheel rotational angle and the wheel steering angle that can also be changed, for example, as a function of the vehicle speed.
Since in steer-by-wire systems a variety of built-in sensors, but in particular also the actuator, which is actuated by signals of the steering wheel input and, as a result, pivots the steerable wheels, can fail, these systems provide a so-called default level, which in view of a desired design flexibility is configured, for example, hydraulically and can include features such as a hydraulic feed unit, which is coupled to the steering wheel, and a hydraulic actuating unit, which is coupled to the steering gearbox and which can form by itself the steering gearbox in a specific embodiment, as well as a hydraulic circuit, which hydraulically connects together these two units.
This hydraulic circuit has at least two hydraulic lines, by which the hydraulic actuating unit, induced by the hydraulic feed unit, is able to pivot the steerable wheels into the one or into the other direction, for which reason both the hydraulic feed unit and also the hydraulic actuating unit exhibit two “working directions,” which are directed opposite to one another, starting, for example, from a center position or zero position. However, the hydraulic feed unit can cause a pivoting movement of the steerable wheels by way of the hydraulic actuating unit, only if between the two hydraulic lines, by which the hydraulic actuating unit can be moved into the one or into the other direction, there is no short circuit. In the prior art such a short circuit is produced by way of a so-called short circuit line, which has a simple shut-off valve, which can be either closed or completely opened, by opening this shut-off valve, when the hydraulic default level is not supposed to be effective. In this case, when the hydraulic feed unit is actuated, the hydraulic medium is merely recirculated from one side of this feed unit by way of the open short circuit line to the other side of this feed unit. In contrast, this shut-off valve, which is called a steering valve, is or remains closed, when the hydraulic default level has to be effective, for example, due to a malfunction of the actuator or by its actuation.
Furthermore, the known steer-by-wire systems have a so-called steering torque simulator, with the aid of which a torque, directed opposite the steering torque generated by the driver, can be applied to the steering wheel, in order to give the driver a typical steering feeling and, thus, a quasi tactile contact with the road. In the prior art this steering torque simulator is formed by an electric motor, which acts on the steering wheel and is suitably actuated by an electronic control unit, so that a so-called hand torque or rather so-called hand forces is (are) adjusted at the steering wheel as a function of the different boundary conditions to be considered.
However, such an independent steering torque simulator in the form of a complex electric motor to be actuated represents an expense that would be desirable to avoid, for which reason the present invention proposes a much simpler vehicle steering system, which can adjust in a targeted way a hand torque that can be sensed by the driver at the steering wheel.
The solution to this problem is characterized in that the steering valve is designed as a proportional servo valve, of which the flow cross section can be adjusted in essence continuously between the two extremes “completely closed” and “completely open.” Even in the event of a functional actuator, this steering valve is not held in its completely open position, but rather in the closed position or is moved into a partially open position, when the intent is to cancel the essentially fixed correlation between the steering wheel rotational angle and the wheel steering angle when the steering valve is closed. The actuator is actuated at least when the steering valve is totally closed in such a way that a desirable so-called hand torque, which the driver can sense and which is a function of at least one boundary condition, adjusts for the respective steering process at the steering wheel.
Working on this basis, the steering system according to the invention may be of the so-called by-wire design type, but is not operated as a by-wire system in the pure form in the bulk of the standard operating points, but rather in a mixed form between a by-wire operation and an activated default level. In this context the hydraulic default level is switched on completely or proportionally as a function of the actuation of the steering valve; at the same time the elements of the so-called by-wire area are activated. Thus, in the presence of a driver's steering request the hydraulic feed unit and the actuator for implementing this steering request work side by side in parallel. As a result, the driver will obtain basically a hand torque at his steering wheel by means of the hydraulic feed unit. In any case when the steering valve is closed and, hence, when the default level is totally activated, the actuator ensures that this hand torque takes on a certain (desired) amount.
If this steering valve, which enables or prevents a short circuit between the two hydraulic lines of the default level, is held in its closed position even if the actuator is functional, then a wheel steering angle can be adjusted as a function of a specified steering wheel rotational angle by means of this default level, and at the same time a desired hand torque at the steering wheel can be adjusted by suitably actuating the actuator. The latter is actuated in such a manner that when the driver turns the steering wheel and the wheel steering angle is adjusted, the steering gearbox causes a build-up of the hydraulic differential pressure in the two hydraulic lines. Induced by the resistance, opposing the steerable wheels at their steering angle, and implemented by the hydraulic feed unit, this differential pressure results in a desired hand torque at the steering wheel. The latter (the hand torque) can be a function of the boundary conditions—that is, in particular the speed of the vehicle.
Since the hydraulic default level, which is present in any event and is necessary for safety reasons, is used to generate a hand torque at the steering wheel, there is no need for an expensive independent steering torque simulator. At the same time, when the steering valve is closed, there exists a fixed correlation between the steering wheel rotational angle and the wheel steering angle, with the exception of deviations that result from the elasticity of the hydraulic transfer between the hydraulic feed unit and the hydraulic actuating unit.
In addition, the steering system according to the invention makes it possible to cancel this aforementioned fixed correlation between the steering wheel rotational angle and the wheel steering angle, so that an additional advantageous property of a steer-by-wire system can be used—that is, a wheel steering angle that deviates from the driver's input can be adjusted, a feature that can be used, as is well known, for example, for stabilizing an unstable driving condition. This feature is achieved in a steering system according to the invention in that the steering valve, which can be designed, of course, not only as a one-way valve, but also as a multi-way valve, can take up not only discrete switching positions in so-called end positions, but also any intermediate position. Consequently, this steering valve is a so-called proportional servo valve, which can produce not only either a 100% short circuit between two hydraulic lines or can totally suppress such a short circuit—as is the case with a valve that enables only two discrete switching positions—but rather such a proportional servo valve, provided between two hydraulic lines, can also produce a partial short circuit (for example, a so-called 50% short circuit) between these hydraulic lines.
If the steering valve is moved, as a function of the situation, into a partially open position, which produces a partial short circuit between the two hydraulic lines between the hydraulic feed unit and the hydraulic actuating unit, then the fixed correlation between the steering wheel rotational angle and the wheel steering angle is cancelled. When such a partial short circuit occurs, only a portion of the hydraulic medium, pumped by means of the hydraulic feed unit in the course of the driver's steering process, flows into the hydraulic actuating unit, whereas the other portion of the hydraulic medium, pumped by the hydraulic feed unit into one of the two hydraulic lines, then flows back to the other side of the feed unit by way of the other of these two hydraulic lines. Thus, when such a partial short circuit is produced, the steering wheel is uncoupled to some extent from the steering gearbox in such a manner that a steering wheel rotational angle input by the driver is only partially transformed into a wheel steering angle, so that consequently the so-called steering ratio between an input of a wheel steering angle and the subsequently adjusted wheel steering angle is changed. While in the case of a completely closed steering valve a steering wheel rotational angle of 90° induces, for example, a wheel steering angle of 5°, it is possible to generate with a steering wheel rotational angle of 90° a wheel steering angle of only 2.5°, when a 50% short circuit is produced by means of the steering valve, as a result of which only half of the amount of hydraulic medium that is displaced by the hydraulic feed unit flows into the hydraulic actuating unit. When such a short circuit between the two hydraulic lines is produced, the counter-torque or hand torque, applied at the steering wheel by the steered wheels by way of the hydraulic actuating unit and the hydraulic feed unit, is correspondingly less.
If, instead of the partial short circuit between the two hydraulic lines, a complete short circuit were to be produced between these lines by opening completely the steering valve, then no hydraulic medium would flow at all from the hydraulic feed unit into the hydraulic actuating unit, so that no torque could be introduced by the steering wheel into the steering gearbox by the hydraulic route. Consequently, no wheel steering angle at all would be produced under the influence of the hydraulic feed unit, and the hand torque, which the driver could feel at the steering wheel, would be exclusively the result of reverse circulation of the hydraulic medium from one side of the hydraulic feed unit to the other side of the same.
However, when a partial short circuit between the two hydraulic lines is produced, then just the desired wheel steering angle can be adjusted by means of the actuator. From the resulting pivoting movement of the steerable wheels a counter-torque can be transmitted proportionally as the hand torque to the driver at his steering wheel as a function of the resulting partial short circuit. It is also possible to adjust at the steering wheel a wheel steering angle according to a suitable arithmetic routine stored in an electronic control unit using the actuator with simultaneous evaluation of the signals of the rotational angle sensor such that the driver does not have to be conscious of this procedure at his steering wheel, after the fixed correlation that exists between the steering wheel rotational angle and the wheel steering angle when the steering valve is closed has been cancelled. In particular, it is then possible to set wheel steering angles that can stabilize a potentially unstable driving condition of the vehicle. To what extent the driver can detect such wheel steering angle adjustments that deviate from his input at his steering wheel depends, in particular, on the size and/or range of the respective short circuit produced with the steering valve.
In principle, then with this engineering measure it is possible to adjust not only a wheel steering angle, which deviates from the driver's steering input, for the purpose of stabilizing the driving condition of the vehicle, but it is also possible with this engineering measure to change the steering ratio, which has already been described in brief above, between the steering wheel and/or a steering wheel rotation angle input with the steering wheel, and the resulting wheel steering angle. If, for example—as described above—the hydraulic medium, which is pumped by the hydraulic feed unit through the first hydraulic line, flows away from the pressure side of this feed unit, while hydraulic medium then flows through the other hydraulic line to this feed unit and, in so doing, can flow back, as it were, to the intake side of the same, then in the case of a so-called 30% short circuit by definition only 30% of the pumped hydraulic medium, flowing into the first hydraulic line, is recirculated back through the second hydraulic line to the intake side of the feed unit. Therefore, in the case of such a 30% short circuit, 70% of the steering input by the driver is implemented via the hydraulic default level, so that without the use of actuator the driver senses the counter-torque, resulting from the corresponding wheel steering angle, as a hand torque at the steering wheel. This hand torque can be reduced to the desired value by suitably actuating the actuator in that this actuator introduces a suitable boost torque into the steering gearbox.
In the case of a so-called 60% short circuit, only 40% of the steering input by the driver is implemented by way of the hydraulic default level, so that without the use of the actuator the driver once again senses the counter-torque, which results from the corresponding wheel steering angle and which is naturally less than in the case of a 30% short circuit under otherwise unchanged boundary conditions, as a hand torque at the steering wheel. Even this hand torque can be reduced to an even lower desired value by suitably actuating the actuator in that this actuator introduces a suitable boost torque into the steering gearbox.
In the case of a so-called 0% short circuit, 100% of the steering input by the driver is implemented by way of the hydraulic default level, so that without the use of the actuator the driver once again senses the counter-torque, which results from the corresponding wheel steering angle and which is higher than in the two preceding cases, as a hand torque at the steering wheel. Once again this hand torque can be reduced to a desired value by suitably actuating the actuator in that this actuator introduces a suitable boost torque into the steering gearbox.
This described adaptation of the steering ratio with simultaneous adjustment of the hand torque by way of the hydraulic default level is possible, if the steering torque or hand torque applied by the driver is directed opposite the steering direction—that is, for example, while parking or steering out of the steering center at a higher vehicle speed. In this case the steering ratio can be reduced by opening the steering valve, because then the hydraulic medium can flow out of that one of the two hydraulic lines, in which a higher pressure level prevails, into the other of the two hydraulic lines, in which a lower pressure level prevails, by way of the steering valve.
In contrast, when the steering movement and the hand torque or steering torque are in the same direction, as is the case, for example, when steering back into the neutral position under the effect of a standard resetting torque that centers the steering, the steering ratio or the hand torque is not adapted arbitrarily, because in this case the pressure differential between the two hydraulic lines is the inverse of the above described case, so that the hydraulic medium cannot be exchanged between the two hydraulic lines in the actually desired direction. If, however, the adjustment of a desired hand torque is of secondary importance—for example, especially in the event of executing vehicle dynamic management measures, where a so-called vehicle dynamic controller adjusts, independently of the driver, a specific wheel steering angle by means of the steering system according to the invention, in order to stabilize the handling of the vehicle—then an arbitrary steering angle can actually be set in the above described way, as in the case of a complete by-wire steering system.
Therefore, these aforementioned three exemplary cases illustrate that in the case of a vehicle steering system according to the invention, the steering ratio between the steering wheel and the related steerable wheels can be adjusted at least in certain driving situations by selecting an arbitrary so-called “short circuit component” by way of the steering valve that is designed as a proportional servo valve, and that at the same time a desired hand torque can be adjusted at the steering wheel by suitably actuating the actuator without having to have for this purpose an independent steering torque simulator. In addition, in extreme driving situations the steering angle and, thus, the steering ratio can be freely superimposed and/or adjusted, if the hand torque does not have to be adjusted in a defined way—that is, for example, in the event of very fast vehicle dynamic management measures with respect to the steering. The result is an extremely simple by-wire steering system, because its necessary (and consequently already existing) default level in interaction with the suitably actuated actuator forms the steering torque simulator. This suitable actuation of the actuator is taken over preferably by an electronic control unit, which provides for this purpose suitable input variables, for example also the differential pressure between the two hydraulic lines between the hydraulic feed unit and the hydraulic actuating unit, as a result of which this differential pressure can be taken into proper consideration even in the course of actuating the short circuit valve.
The hydraulic feed unit and the hydraulic actuating unit can be configured in the form of various design types known from the prior art. However, at the same time it ought to be possible to shift an element of this unit into both a first direction and also a second opposite direction, in order to enable a rotation or pivoting movement in both directions starting from a center position or zero position of both the steering wheel and the steerable wheels. For example, a hydraulic reversing motor can be used as such a hydraulic actuating unit, and a reversing pump, as the hydraulic feed unit. As an alternative suitable piston-cylinder units can be used.
Similarly, a variety of design types that are known from the prior art can be used for the steering gearbox and the actuator. In a first preferred embodiment, the actuator can be designed as an electric motor with a downstream transmission, which acts on the steering gearbox, which has a longitudinally shiftable gear element, of which both ends can be hinged to the tie rods assigned to the two steerable wheels. In this case the steering gearbox itself can be an essentially standard rack and pinion steering gearbox, to which is connected in series, as it were, a hydraulic actuating unit (for the default level) in the form of a cylinder-piston unit. In one possible alternative embodiment, the actuator can be designed as an electric motor that acts between the steering wheel and the hydraulic feed unit (as is also shown by one of the embodiments that are explained below with reference to the diagrammatic drawings).
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.